1. Field of the Invention
The invention relates generally to minimization of fuel consumption in multi-mode vehicles.
2. Description of the Related Art
Modern vehicles that move on land, on sea, or in the air frequently have power plants with multiple modes. A truck can be in various gears, each of which is a mode. A multi-engine aircraft will sometimes deliberately operate with some of its engines shut down. Each set of operating engines is a mode. Ships sometimes have multiple shafts, or multiple engines of different types, and a decision must be made regarding the best mode to use. Fuel consumption varies continuously with speed in each mode, and each mode has limits on the speeds that can be safely obtained.
The problem of mode selection is particularly acute for navy ships, because they must be capable of high speeds. The high-speed mode may be very different and less efficient than other modes, but must still sometimes be employed for tactical reasons.
Naval surface-combatant ships consume large quantities of fuel. For example, in Full Power mode four LM2500 gas-turbine engines power a DDG51 Arleigh Burke-class, guided-missile destroyer. Other modes where some of these engines are shut down, or where one of the DDG51's two shafts is not powered at all, are also possible. Depending on propulsion mode and speed, these four turbines can collectively consume from 600 to 7,000 gallons per hour (GPH) of distillate marine fuel. That means that the destroyer's fuel consumption while at sea is between one hundred thousand and one million gallons per week. For surface combatants alone, marine fuel costs the U.S. Navy about a billion dollars per year. Even a small percentage change in such quantities can have significant fiscal consequences.
The U.S. Navy is keenly aware of its fuel consumption, seeks innovations to conserve fuel, and prescribes how its ships should monitor, control, and minimize fuel use. For each ship class, sea trials are conducted to determine fuel-consumption as a function of speed for every propulsion-plant mode, and then published as standards in tabulations that can be used to plan transits and ship plant operating modes. Mixed-mode operations, however, are not included in those tabulations.